1. Field of the Invention
The present invention relates generally to seismic signal processing and, more particularly, to apparatus and methods for improved interpretation of seismic data to more accurately identify geopressures, seismic velocities, and/or other formation attributes.
2. Description of the Background
When planning the drilling of a well, the drilling engineer must make decisions about the mud program, the casing program, and the like. The consequences of such decisions are significant and impact the likelihood of success of the well, the cost of the well, the environment, and even the safety of the well personnel. For instance, if the mud selected or used at any particular depth is too light to hold the actual well pressure, or if other pressure control equipment utilized is inadequate, then blowouts, loss of life and property, environmental damage, and the like may occur. Costs of blowouts may also include environmental remedial costs, loss of production, and relief well costs. If a blowout occurs, well control specialists can be quite expensive, costing millions of dollars per well. On the other hand, if the mud selected is too heavy, then loss of circulation may occur with costly consequences such as kicks, underground blowouts, stuck pipe, and the need for side tracks. As well, damage to the formation may occur that may reduce or limit the well""s productivity. Additional control equipment for handling pressure is costly and the over-engineering of a well can add significantly to the well costs. If the anticipated pressure does not materialize, then this money is wasted.
It is therefore desirable to keep drilling mud weights as light as possible to most economically penetrate the earth. It is also desirable to know what pressures are expected so that suitable but economically reasonable pressure control equipment is available on site. The required mud weight will typically vary with depth during the drilling of the well. For the reasons discussed above, mud weight is carefully monitored and may be increased during drilling operations to compensate for geopressure.
It is often desirable to set casing in a borehole immediately prior to encountering a pressured formation and then to increase mud weight for pressure control during further drilling. Setting a casing string which spans normal or low pressure formations permits the use of very heavy drilling muds without risking breaking down of borehole walls and subsequent lost mud. On the other hand, should substitution of heavy drilling mud be delayed until the drill bit has penetrated a permeable overpressurized formation (e.g., sandstone), it may be impossible to remove the drill string without producing a blowout or otherwise losing the well.
Geopressure conditions conducive to blowouts occur, in general, if fluids become trapped in rock and must support some of the overburden weight. As an example, there may be an earth formation of high porosity and high permeability, or a series of such formations, within a massive shale formation that is relatively impermeable. Fluid pressure of fluids trapped within such highly permeable formations (which usually are sands) may increase as the weight of overburden increases during sedimentation above the shale formation. When such formations are penetrated, the large pressure gradient into the borehole can easily result in a blowout.
To make the well program decisions, the drilling engineer typically relies on data from offset wells, assuming offset wells have been drilled. If there are no offset wells, then the decisions may be made in the light of significant geologic uncertainties. Even if there are offset wells, assumptions are made to the effect that the future well will be like the offset well. However, drilling rarely goes just like the offset in spite of the best correlation efforts. For instance, pore pressures, and the attendant possibility of blowouts and/or extra gas production, can vary significantly within even the same field and within close distances of offset wells. Gas pockets with high pressures may be encountered in one well that are not present in a closely adjacent well. For instance, in some fields that normally have low geopressures, gas pockets may exist but may be rare so that when encountered the rig crew may be unprepared.
Well logs are sometimes used to analyze offset wells to determine, for instance, pore pressures at the offset well in the hope that the well to be drilled in the future will have the same properties. As discussed above, due to differences from well to well, the results may not be sufficiently accurate or similar enough to avoid significant problems, set casing at the appropriate positions, and/or to efficiently produce the hydrocarbons. The analysis for pore pressures involves use of well logs such as gamma ray, velocity, and spontaneous potential well logs to select corresponding shale points in a sonic or resistivity well log. A normal compaction trend line is then determined and the departure of the sonic or resistivity data from the trend line is used to compute pore pressures.
From the drilling engineer""s point of view, it would be very highly desirable to accurately predict pore pressures from seismic data. In this way, the data could be specific to each well rather than after the fact and typically a variation from the values predicted by offset wells. Therefore significant efforts and attempts have been made in the past by those skilled in the art to utilize seismic data for such purposes. Unfortunately, attempts to predict pore pressure from seismic data have been relatively poor. In the prior art, seismic velocities are determined from seismic data utilizing normal move out techniques. From the seismic velocities so determined, as discussed in the subsequently listed patents, the normal compaction trend lines have been determined and pore pressures computed. However, while general trends may be seen from such analysis, the seismic velocity determined by prior art seismic data analysis techniques does not have sufficient resolution accuracy to permit actions to be taken at optimal well depths to most efficiently drill and produce the well.
The following representative patents show attempts to utilize or improve upon the move out velocities derived from seismic data for determining pore pressure as per the prior art:
U.S. Pat. No. 3,898,610, issued Aug. 5, 1975, to E. S. Pennebaker, Jr. discloses methods of geopressure assessment in an area proposed for drilling: first, perform a seismic observation (using a common midpoint (CMP) method as illustrated in Pennebaker FIG. 1) to determine average seismic velocity as a function of depth by move out techniques. Next, compute interval transit time as a function of depth, and then compare these observed interval transit times to putatively normal interval transit times as illustrated in Pennebaker. Depths where observed interval transit times are greater than normal indicate lower-than-normal velocity and inferentially greater-than-normal porosity and thus geopressured fluids. Putatively normal interval transit times are either (I) directly measured in a borehole in the general area which encountered only normal pressures during drilling or (ii) computed by following an expression for seismic velocity V (feet/second) as a function of depth D, with D measured in feet from a location of known seismic velocity.
U.S. Pat. No. 6,374,186B1, issued Apr. 16, 2002, to Dvorkin et al., discloses a method for overpressure detection and pore pressure change monitoring in subsurface gas, liquid hydrocarbon, or water reservoirs from compressional- and shear-wave measurement data. As part of this method, one or more Poisson""s ratios are determined from field-based measurement data and are then compared against known Poisson""s ratio values representative of the particular subsurface formation type. By applying a Poisson""s ratioxe2x80x94pore pressure criterion that is appropriate for that type of formation, an overpressure in the formation is identified.
U.S. Patent No. 5,343,440, issued Aug. 30, 1994, to Kan et al., discloses a method wherein seismic data is combined with well log data to generate a two-dimensional geopressure prediction display; this permits deviated and horizontal well planning plus lithology detection. Shale fraction analysis, compaction trend, and seismic velocity may be automatically or interactively generated on a computer work station with graphics displays to avoid anomalous results. Corrections to velocity predictions by check shots or VSP, and translation of trend curves for laterally offset areas increases accuracy of the geopressure predictions. Multiple well logs in a basin permits analysis fluid migrations.
U.S. Pat. No. 5,937,362, issued Aug. 10, 1999, to Lindsay et al., discloses a pore pressure prediction method that includes the steps of: (a) designing a normal compaction trend velocity model; (b) testing the normal compaction trend velocity model; (c) designing 3-D spatial adjustment parameters to compensate for water depth; and (d) processing a 3-D velocity field using the interpreted normal compaction trend velocity model and the 3-D spatial adjustment parameters.
The above discussed prior techniques are fraught with numerous potential errors due to the limitation of prior methods of utilizing seismic date to calculate velocities. Consequently, there remains a long felt need for improved methods of seismic signal analysis for more accurately predicting pore pressures It would also be desirable to more accurately determine velocities and/or other attributes from seismic data without the need to rely on move out techniques. Those skilled in the art have long sought and will appreciate the present invention which addresses these and other problems.
It is an object of the present invention to provide an improved seismic analysis method.
It is yet another object of the present invention to predict pore pressures, stress, fracture gradients, and formation compressive strengths from seismic data.
It is yet another object of the present invention to provide means for determining seismic velocities and other formation attributes which avoid the limitations of prior art techniques based on move out techniques.
An advantage of the present invention is more accurate and improved location and description of geopressures.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. However, it will be understood that above-listed objectives and/or advantages of the invention are intended only as an aid in quickly understanding aspects of the invention, are not intended to limit the invention in any way, and therefore do not form a comprehensive or restrictive list of objectives, and/or features, and/or advantages.
In accord with the present invention, a method of analyzing seismic data is provided. In one possible embodiment thereof, the method is utilized to determine geopressures for an earth formation. The method may comprise one or more steps such as, for instance, obtaining seismic data related to the earth formation wherein the seismic data may comprise one or more seismic signals transmitted through the earth formation, and/or determining at least one frequency related attribute of the one or more seismic signals, and/or determining geopressures from the at least one frequency related attribute.
The frequency related attribute may be any frequency related attribute for one or more seismic signals such as, for example only, an average frequency, a frequency attenuation, an instantaneous frequency, a signal amplitude within a frequency band of the one or more seismic signals, a frequency related transform, a shape of a frequency response envelope, and/or other frequency related attributes or combinations, derivations, or permutations thereof.
The method may further comprise plotting geopressure with respect to time/depth within the earth formation. It is understood that time and depth are interchangeable for seismic data and seismic data may be displayed in either format and therefore may be referred to as time/depth herein. Other steps may comprise determining the geopressure by calibrating the frequency related attribute in terms related to geopressure and/or determining the geopressure as a function of the frequency related attribute.
In another embodiment, the method may comprise determining at least one of a velocity or a geopressure related attribute of the one or more seismic signals from the at least one frequency related attribute. In this embodiment, the frequency related attribute may be calibrated or utilized to derive the seismic velocity or the geopressure related attribute.
In another embodiment, a method for determining an overpressure may comprise one or more steps such as, for instance, determining a range of values for geopressure with respect to time/depth, determining a rate of change of a maximum of the range of values, and determining a time/depth wherein a transition in the rate of change occurs such that the rate of change stops increasing and decreases.
Other steps may comprise locating a formation at a time/depth below the transition having a lithology wherein fluid may flow. For instance, the formation could be a sand or limestone located below a large shale formation. In one presently preferred embodiment, the step of determining a rate of change comprises determining a derivative of the maximum of the range of values.